1. Field of the Invention
The present invention relates to an image reading apparatus which captures images on a document, and more specifically, an image reading apparatus which can capture images on both front and back sides of the document by single transfer.
2. Description of the Related Art
Conventionally, as reading apparatuses such as a copying machine, a facsimile and a scanner for computer inputs, image reading apparatuses (automatic double side reading apparatuses) have been used which automatically read image information on both front and back sides of a document without intervention of users. Among these automatic double side reading apparatuses, a method of reading by inverting the front and back sides of a document at a document inverting unit is widely used. When inputting image information by front-back inversion, after images on the front side are read by a specific document reading unit, this document is inverted and conveyed again to this specific document reading unit and images on the back side are read. However, in this automatic double side reading by method of front-back inversion, since it is required that the document is inverted after being ejected and conveyed again to the document reading unit, this double side reading takes a long time, and this deteriorates the yield of double side reading. Therefore, for example, there is an art which makes it possible that both sides of a document are automatically read by single transfer of the document without front-back inversion by providing image sensors on both front and back sides of a document path in which the document is conveyed.
In addition, in conventional image reading apparatuses, a method in which a document is irradiated with light from a light source using, for example, a fluorescent light, and light reflected from the document is read by an image sensor through a minification optical system, is widely used. As an image sensor using this method, for example, a one-dimensional CCD (Charge Coupled Device) sensor is available, which simultaneously processes one line. In this method, when reading of one line in the line direction (main scanning direction) is finished, the document is moved for a short distance in the direction (vertical scanning direction) orthogonal to the main scanning direction and the next line is read. This operation is repeated for the entire document size to complete reading of one page of the document. In addition, as a method for successively reading in the vertical scanning direction without movement of the document, it is also possible that reading in the vertical scanning direction is successively carried out by moving a plurality of mirrors by movers such as full-rate carriages or half-rate carriages.
In this reading method, as mentioned above, it is necessary that a document is irradiated with light from a light source and reflected light is read by a CCD sensor through some mirrors, so that the whole unit tends to be large in size. Particularly, in a case where a plurality of image sensors must be provided for reading both sides of a document without inversion, it is difficult to provide such a plurality of CCD sensors due to limitations of space. Therefore, in order to solve such a spatial problem, it has been considered that a small-shaped LED (Light Emitting Diode) is used as a light source and an image sensor called CIS (Contact Image Sensor) which reads images by, for example, a linear sensor through a selfoc lens, is used.
Generally, emission spectrum (relationship between an emission wavelength and emission intensity) greatly differs between an LED and a fluorescent light. Therefore, when a fluorescent light is used as a light source of one reading unit and an LED is used as a light source of the other reading unit, the difference in emission spectrum between the fluorescent light and the LED poses the following problem when color images formed on both sides of a document are read as monochrome images.
For example, a case where a spectral reflection of an image formed on a document has an inherent tendency (for example, including many images in blue) is considered. In the case, in a reading unit using a fluorescent light as a light source, when blue components are included in the emission spectrum of the fluorescent light, reading images corresponding to the densities of blue images can be outputted. On the other hand, in a reading unit using an LED as a light source, when blue components are rarely included in the emission spectrum of the LED, reading images corresponding to the densities of the blue images cannot be outputted, so that reading signals corresponding to high density are normally outputted. Namely, due to the difference in emission characteristics between the light sources, the density obtained with respect to the blue image differs between the front and back sides. This problem may also occur in cases where the image includes many components of other colors as well as the image including many blue components.